In the real world would there be a significant difference between the amount of light and details gathered by the Nikon D5100 vs the Canon 550D sensors?. Esp for night sky.. Star Trails Milky way and stuff?

I know that the D5100 has around 10% bigger sensor than the 550D but there seems to be much more info/support using Canon DSLRs for night sky photography out there.

I have also read somewhere that high pixel density can make sensor less sensitive or have some -ve impact on overall picture...

In theory, yes a higher pixel density can be detrimental but I don't think you can go wrong with either though I suspect the D5100 has a slight edge. With that said, I would rather use a 550D with a great lens/telescope than a D5100 with a decent lens/telescope for astrophotography.

It's perhaps worth bearing in mind that critics of a higher pixel density neglect to mention that the pixels' light sensitivity tends to be improved from those in previous sensors so it would be a bold if not naive statement to say that more pixels absolutely means more noise.

It's personal preference, but I prefer the Live View on the Canons for focusing astro shots AND you also get the free EOS utility which lets you remote control the bodies with a laptop over USB and program in a set of long exposures using the intervalometer function. There's also lots of people experienced in modding the bodies for astro work.

Canon generally is the preferred choice for astrophotography over Nikon, in the past they were highly acclaimed for having much better high ISO performance. Today, there's little difference (Nikon's actually generally have better high ISO performance, the D5100 beats the 550D), but you may prefer Canon's live view system, and the ability to control the camera from your computer is an advantage. Canon also has a better selection of telephoto lenses if you do a lot your work with tele lenses, but Nikon's selection is by no means lackluster.

All that aside, I'm still able to get excellent shots out of my D7000 of the night sky, and using the 35mm f/1.8 DX lens (a lens that Canon doesn't really have an equivalent to) is a treat. I'll try to post a few shots that I got of the Milky Way later as examples.